Polycation salts of the polyoxy anions are generally insoluble or poorly soluble. Polyoxy anions are anions that contain at least one non-oxygen atom and two or more oxygen atoms. Examples of polyoxy anions include phosphate, phosphite, sulfate, sulfite, nitrate, borate, arsenate, or selenate. Polyvalent cations are cations that have a charge of plus two (+2) or more in the ionic form. Examples of polyvalent cations include the ionic forms of calcium, magnesium, iron, zinc, manganese, copper, aluminum, and cadmium. Calcium, magnesium and other polyvalent cation phosphate salts are generally insoluble or very poorly soluble. According to the Handbook of Chemistry and Physics: calcium hypophosphate (Ca2P2O6.2H2O) is insoluble, calcium metaphosphate (Ca(PO3)2) is insoluble, calcium orthophosphate {di-sec.} (CaHPO4.2H2O), also known as natural brushite, is poorly soluble at 0.0316 grams per 100 grams of water at 38° C., calcium orthophosphate {mono-prim.} (Ca(H2PO4)2.H2O) is soluble at 1.8 grams per 100 grams of water at 30° C., calcium orthophosphate {tri-tert.} (Ca3(PO4)2), known as natural whitlockite, is soluble at 0.002 grams per 100 grams of water, calcium pyrophosphate (CaP2O7) is listed as insoluble, and calcium pyrophosphate pentahydrate (CaP2O7.5H2O) is listed as slightly soluble. Other polyvalent cations, such as, but not limited to, calcium, magnesium, iron, zinc, manganese, copper, aluminum, and cadmium, form phosphate salts that follow a similar pattern of insolubility or very low solubility. The polyvalent cation salts of sulfate follow a similar pattern as do the polyvalent cation salts of other polyoxy anions.
The very limited solubility of calcium and magnesium phosphate salts is commonly recognized and has been used commercially in detergents wherein phosphate compounds were added as water softeners to remove the calcium and magnesium from the solution since calcium and magnesium interfered with the action of the active detergent agents. This use of phosphate is referenced in U.S. patents filed as early as 1942, for example, in U.S. Pat. No. 2,385,929, to Meltes, which is hereby incorporated by reference.
However, formation of poorly soluble polyvalent cation-polyoxy anion salts is a problem in agriculture. Irrigation of crops occurs in all parts of the world. It is common to apply fertilizers in the irrigation water or as a foliar application during the growing season. A problem arises when applying phosphorous containing fertilizers with “hard water.” Hard water is water that contains polyvalent metal cations, calcium and magnesium being the most common. Adding phosphorous to water containing even a modest amount of calcium, magnesium, or other polyvalent ions can result in the formation of poorly soluble or insoluble phosphate salts. Calcium phosphate, in the form of brushite, is the most common. The creation of poorly soluble phosphates drastically reduces the fertilizer effectiveness of phosphorous additions. It can also result in these same phosphate salts precipitating and clogging the fertilizer application system components, such as nozzles, spray tank filters and screens, tubing, sprinklers, or drip irrigation emitters, thereby making it impossible to apply phosphate fertilizer using these methods. In these circumstances, the farmer has to find another way to apply the necessary phosphorous to his crops during the growing season or, as is more common, apply all of it into the soil before planting.
Many of the waters used for irrigation in the United States are considered “hard water,” which refers to water having a high mineral content. The hardness of water is determined by the concentration of multivalent cations in the water. The U.S. Geological Survey (USGS) has created general guidelines for classification of the hardness of waters: 0 to 60 mg/L (milligrams per liter) of calcium carbonate equivalents is classified as “soft”; 61 to 120 mg/L is classified as ‘moderately hard’; 121 to 180 mg/L is classified as “hard”; and 181 mg/L and higher is classified as “very hard.” The molecular weight of calcium carbonate is 100.9 and the molecular weight of calcium is 40.08. Therefore, the hardness values convert as follows: 0 to 0.59 mM Ca (millimoles Ca/Liter) is “soft”; 0.60 to 1.19 mM calcium is “moderately hard”; 1.20 to 1.78 mM calcium is “hard”; and 1.79 mM calcium or more is “very hard.”
Some irrigation waters are “very hard.” For example, irrigation water from the Colorado River, a major source of irrigation water in the southwestern United States, is reported to be “very hard,” with a calcium carbonate equivalent of 280 mg/L. The hardest waters (greater than 1,000 mg/L) have been measured in streams in Texas, New Mexico, Kansas, Arizona, and southern California.
Also, under similar conditions of hardness, poorly soluble calcium sulfate (gypsum) and other poorly soluble sulfate salts may form and reduce fertilizer efficiency and can even cause plugging of water applying devices of irrigation systems, such as spray nozzles, spray tank screens, sprinklers, microsprinklers, drip tape emitters, etc.
The problem of precipitate formation in irrigation systems when agricultural chemicals are mixed with hard water has been recognized for decades. For example, in U.S. Pat. No. 3,592,386, to Tschudy, Jr., which is hereby incorporated by reference, the inventor notes, “For example, an especially acute precipitate problem arises when pesticides or fertilizers containing phosphate compounds are admixed with hard irrigation water.” The solution taught by Tschudy, Jr., comprises a method that includes forming separate but converging streams of irrigation water and phosphate solution, wherein the streams converge externally from the stream-forming means.
In U.S. Pat. No. 5,997,602, to Äijälä, which is hereby incorporated by reference, the inventor notes, “If such a concentrate solution/fertilizer solution contains calcium and/or magnesium salts and ordinary water-soluble phosphorus sources such as for instance monoammonium phosphate, diammonium phosphate or potassium phosphate, the pH of the solution will be so high that calcium forms insoluble phosphate salts. The insoluble component precipitates, thus clogging the irrigation system, and its nutrients are no longer in a form which the plants can utilize.” Äijälä teaches “a stable, concentrated aqueous fertilizer suspension, which can be diluted with water to form a working solution, said suspension containing 10 to 60% by weight of water and 40 to 90% by weight of a plant nutrient composition.”
In U.S. Pat. No. 7,569,091, to Peters, which is hereby incorporated by reference, the problem is again acknowledged; “Compounding this difficulty of determining the proper amounts or concentrations of multiple nutrients in compound fertilizers is the fact that nutrients when blended together can interact with each other, with counter ions, or with the solvent, normally water, in untoward ways.” This is an even more significant problem in areas having ‘hard’ irrigation water; i.e., water with high ambient concentrations of Ca or Mg. The resulting precipitates can remove free nutrient from use by the plant and clog irrigation and spraying equipment.”
Conventionally, fertilizers, for example, phosphates and sulfates, may be either pre-mixed with soft water prior to deliver to the user, or may be provided in concentrated form and mixed by the user with a deionized or soft water. In either case, the costs and inconvenience of shipping and handling large quantities of fluids is undesirable.
There remains a need to provide fertilizer compositions that can be used in conventional irrigation systems and that can be delivered with hard water without forming precipitates that can clog such irrigation systems.